If you are not losing brake fluid, the master cylinder is about the only suspect. Besides just failing on its own, this happens real often when the driver is surprised by a sudden leak, or when do-it-yourselfers pedal bleed with a helper. Any time the brake pedal is pushed over half way to the floor, the rubber lip seals in the master cylinder are run over the crud and corrosion that build up in the lower halves of the bores. That can rip them and cause a low or slowly-sinking pedal. Often that damage does not show up until two or three days later.
Monday, March 29th, 2021 AT 10:52 AM